Numerical study of the evaporation and thermal decomposition of a single iron(III) nitrate nonahydrate/ethanol droplet

Abstract

The heating, evaporation, and possible thermal decomposition of spherically symmetric droplets consisting of a precursor solution of iron(III) nitrate nonahydrate (INN) and ethanol (EtOH) for nanoparticle synthesis in spray flames are studied numerically. The multicomponent droplets are at standard conditions and the evaporation is initiated through the elevated temperature of the convective ambience. The liquid mixture properties of INN and ethanol used in the model are fits to new experimental data. In general, there are two different pathways through which the nanoparticle synthesis may occur from the precursor solution. In one pathway, the particle forms directly inside the liquid precursor solution, and in another pathway, the droplet is transferred entirely into the gas phase from which the nanoparticle may form. The process is initially governed by the evaporation of ethanol and of the water from the INN. For ambient gas temperatures below the thermal decomposition temperature Tth of INN, the evaporation eventually leaves behind an iron(III) nitrate particle and for higher temperatures beyond Tth, thermal decomposition and liquid-phase reactions may occur inside the precursor droplet, transferring the iron(III) nitrate entirely into gaseous Fe2O3 and N2O5, so that the entire liquid droplet transforms into the gas phase. Parameter studies of the evaporation of the INN/ethanol droplet for both situations are presented. Moreover, the process in both dry and humid air is investigated. The water condensation on the droplet surface prolongs the droplet evaporation time and leads to higher droplet surface temperatures for high ambient gas temperatures, promoting thermal decomposition of the iron(III) nitrate.